This invention relates generally to electronic power supplies and in particular to a circuit for discharging charged capacitors in a power supply after a.c. power is removed.
The typical d.c. power supply used to supply d.c. voltages from an a.c. line voltage operates by converting the a.c. line voltage level to a desired a.c. voltage level using a transformer, rectifying the a.c. voltage to a pulsating d.c. voltage with semiconductor diodes, and filtering the pulsating d.c. voltage with capacitors to obtain the smoothed d.c. voltage. Capacitors, by virtue of their ability to store large amounts of electrical charge, function as filters by alternatively storing the charge from the peaks of the pulsating d.c. voltage which functions as the charging voltage and then releasing the charge between the peaks in the manner of a reservoir.
Capacitors have the ability to retain a substantial amount of stored charge long after the a.c. line power is removed if there is no path for the charge to bleed off, presenting a safety hazard to persons who may accidentally come in contact with the power supply. To address this concern, various techniques of discharging the capacitors have been devised. The simplest technique involves coupling a bleeder resistor in shunt across the capacitor. After the charging voltage is removed, the capacitor discharges through the bleeder resistor which dissipates the stored charge as heat. The disadvantages of the continuously-coupled bleeder resistor are that it continually dissipates energy, resulting in wasted energy which appears as heat build-up in the power supply and heavier duty components must be used to dissipate the heat continuously. An improved technique is to connect a relay in series with the bleeder resistor with the relay coil coupled to the a.c. line voltage in such a way that the relay contacts close and couple the bleeder resistor to the capacitor when the power supply is switched off. In this way, no power is dissipated in the bleeder resistor during the normal operation of the power supply. Relays that are capable of this task tend to be bulky and expensive and, as mechanical switching devices, may not achieve adequate reliability where there is a concern for safety. Semiconductor switches that perform the function of the relay have lacked a simple means of detecting the removal of the charging voltage from the capacitor when the power supply is switched off. Therefore, it would be desirable to provide an active discharge circuit using a relatively small number of inexpensive semiconductor components that is capable of detecting the removal of charging voltage and responsively coupling an energy dissipating element across the filter capacitor to facilitate its safe discharge.